collar locator

A collar locator (also abbreviated CCL for casing collar locator) is a downhole logging tool or sub that detects the electromagnetic anomaly produced by each casing or tubing collar — the coupling connecting adjacent pipe joints — as the tool passes through a cased wellbore, generating a distinctive voltage spike at each collar position that is recorded on the collar log depth track and used to correlate wireline or coiled tubing tool depth against the known joint tally from the casing running record, establishing a cased-hole depth reference accurate to 0.05 to 0.15 m that is independent of wireline cable stretch and temperature-induced measurement errors accumulating over thousands of meters of wellbore depth. The collar locator is the foundational depth-reference instrument for every cased-hole completion and intervention operation — perforating, packer setting, plug placement, recompletion, and mechanical workover — because the alternative (relying solely on cable-measured depth at surface) accumulates errors of 1.5 to 6 m in WCSB wells at 1,500 to 3,500 m depth from cable stretch, temperature stretch, and sheave-wheel calibration errors that individually are minor but collectively can displace a perforating gun from its target interval entirely. In the Western Canada Sedimentary Basin, collar locators are deployed in three primary configurations: as a wireline-conveyed GR-CCL combination tool (gamma ray plus casing collar locator on the same tool string) run in cased-hole before perforating or recompletion to correlate formation markers visible on the open-hole gamma ray log with casing collar positions in the completed wellbore, allowing perforation depth to be specified as an offset in meters from a known collar number referenced to the casing tally; as a CCL sub integrated into coiled tubing-conveyed plug-and-perf BHA strings in WCSB Montney and Duvernay horizontal completions at 1,500 to 3,500 m depth, where the CCL spike count from the casing shoe provides a continuous depth check against the CT encoder wheel measurement at each stage interval to ensure plug setting and gun firing occur within the plus or minus 1 to 3 m depth tolerance required for effective hydraulic fracture stage isolation; and as a CCL channel integrated into production logging tool strings deployed in WCSB Cardium, Viking, and Devonian producer wells during reservoir surveillance and recompletion operations, providing a continuous depth reference throughout the logging run that ties every production anomaly to a specific formation interval identified on the original open-hole log suite.

• Electromagnetic detection principle, sensor design, and CCL signal characteristics in WCSB cased-hole operations: The collar locator sensor consists of a permanent bar magnet (typically Alnico or rare-earth magnet for higher field strength in small-OD tools) mounted in the tool body with one or two induction pickup coils wound around a soft-iron core at the magnet's pole faces; as the tool traverses the borehole, the magnetic field permeates the surrounding casing steel with a flux density proportional to the local steel cross-sectional area. At a standard API coupling collar, the threaded coupling body adds 10 to 30 mm of additional steel radial thickness around the full circumference, increasing the cross-sectional steel area by 40 to 80 percent relative to the adjacent pipe body and inducing a proportional flux perturbation in the pickup coil as the tool enters and exits the collar zone. The resulting voltage waveform is a characteristic doublet (positive-negative or negative-positive depending on tool polarity and direction of travel) with amplitude in the 50 to 500 mV range for standard API casing weights, decaying to background within 0.2 to 0.5 m of the collar centerline; collar signal amplitude scales with the mass contrast between the coupling and the pipe body, so heavy-wall casing (54 kg/m P-110) produces stronger CCL spikes than standard-weight pipe (23 kg/m N-80) in the same OD, providing more reliable detection in thick-wall WCSB Foothills casing strings.
• GR-CCL combination log as the standard WCSB cased-hole depth correlation workflow for perforating and recompletion: The gamma ray-casing collar locator combination is the most frequently run wireline log in WCSB cased wellbores because it provides two independent depth-correlation channels simultaneously: the CCL spike train positions the tool relative to the physical casing construction (collar count from the shoe), while the gamma ray log provides formation lithology response (shale-carbonate-sand contrast) that can be directly overlaid on the original open-hole GR to confirm that the cased-hole tool is at the same formation depth as the open-hole formation evaluation. In WCSB Devonian carbonate recompletions at Pembina Nisku, Leduc, and Swan Hills, the GR-CCL combination log is run with a 1:200 depth scale to distinguish individual reef and off-reef carbonate cycles that are 1 to 5 m thick, with perforation depth specified as a distance in meters below a named collar (for example, "perforate 3.2 m below collar 47 counting from the casing shoe") rather than an absolute measured depth, because this collar-referenced depth specification eliminates the systematic cable-stretch component of the depth error that grows with depth and is not reproducible between logging runs on different days.
• CT-conveyed CCL in WCSB Montney and Duvernay plug-and-perf horizontal completions for stage depth accuracy: In WCSB Montney and Duvernay horizontal completions with 20 to 60 stages per well at measured depths of 1,500 to 4,500 m, the CT-conveyed plug-and-perf BHA includes a CCL sub immediately above the plug-and-gun assembly to provide continuous collar count verification as the BHA is pumped and pushed to each stage interval; the CT encoder wheel at the injector head measures cumulative CT movement from the wellhead with an accuracy of plus or minus 0.3 percent of measured depth (typically plus or minus 4 to 13 m over 1,500 to 4,500 m), while the CCL sub detects individual casing collars at known spacing (40.5 m joint length for 5-1/2" casing in most WCSB horizontal wells) to provide an independent depth check at every collar crossing. WCSB completion engineers specify a maximum permissible depth discrepancy between the CCL-derived collar count and the encoder depth of 3 m at each stage target; if this tolerance is exceeded at any stage, the BHA is repositioned before the plug is set, preventing the frac plug from being placed 3 to 5 m from the intended inter-stage barrier location that would compromise hydraulic isolation between adjacent fracture stages and reduce stimulated reservoir volume.
• Depth accuracy, cable stretch corrections, and CCL limitations in WCSB deep wells and deviated wellbores: CCL depth accuracy in WCSB vertical and near-vertical wells (inclination below 40 degrees) is typically 0.05 to 0.15 m per collar, yielding absolute depth accuracy of 0.5 to 1.5 m at 3,000 m depth when the collar count and joint tally are properly correlated from the casing shoe reference; this is 3 to 4 times more accurate than relying on cable-measured depth alone, which accumulates 0.05 to 0.15 percent stretch error (1.5 to 4.5 m at 3,000 m) from cable elasticity, temperature-induced cable length change (steel cable elongates approximately 0.001 m per meter per 100 degrees Celsius temperature change), and sheave wheel eccentricity. In WCSB deviated and horizontal wells above 60 degrees inclination, CCL spike amplitude becomes unreliable because the tool rests against the low side of the casing bore at a standoff from the pipe wall determined by tool OD and casing ID, altering the magnetic flux path geometry and reducing the mass contrast signal amplitude; in WCSB Montney horizontal wells at 90 degrees inclination with 5-1/2" casing (ID 121 mm), a 1-5/8" CCL tool (OD 41 mm) rests against the low side with 40 mm radial standoff, reducing CCL spike amplitude by 30 to 50 percent compared to the vertically centered tool position but not eliminating the signal for standard API coupling collars.
• CCL integration with MWD gamma ray and encoder depth in WCSB multi-zone plug-and-perf quality control: Modern WCSB Montney and Duvernay plug-and-perf completion quality control programs use a three-channel depth verification system combining CT encoder depth, CCL collar count, and CT-conveyed gamma ray correlation to formation markers from the original LWD log; the three channels are displayed simultaneously on the completion engineer's real-time wellsite screen and logged in the job data file, with automated depth discrepancy alerts flagging any mismatch above 2 m between the encoder and CCL depth estimate before a plug set command is authorized. Post-job depth correlation reports comparing the CCL-confirmed plug set depths to the original LWD gamma ray markers are required by WCSB Montney and Duvernay operators before hydraulic fracture pump schedules for each stage are authorized, because a plug placed more than 3 m from the designed stage boundary can create hydraulic communication between stages (short-circuiting the frac) or leave a gap of unstimulated reservoir between stages that permanently reduces well productivity.

WCSB Montney CCL Depth Check Preventing Misperforations at Stage 14

During a 28-stage Montney plug-and-perf completion in northeast British Columbia at 3,240 m total depth, the CT encoder indicated the stage 14 plug-and-gun assembly had reached the designed set depth at 2,890 m measured depth. The CCL sub simultaneously showed the tool positioned 1.8 m shallower than the encoder depth based on collar count from the casing shoe (collar 71 at 2,888.2 m tally depth versus encoder reading of 2,890.0 m). The 1.8 m discrepancy exceeded the operator's 1.5 m tolerance threshold, triggering a repositioning check: the CT was advanced 1.8 m and the CCL confirmed the BHA was now at collar 71 plus 1.8 m offset, matching the tally. The plug was set at the CCL-corrected depth. Post-completion review confirmed the encoder had accumulated a 1.8 m stretch error over 2,890 m due to elevated wellbore temperature softening the CT, with the CCL providing the corrective reference that kept stages 14 through 28 within the 1.5 m depth tolerance and within the target Montney siltstone interval.

Related Terms

Collar log is the recorded output trace produced by the collar locator tool; the CCL spike train on the depth track provides the casing joint tally reference used to correlate wireline and CT tool depth to formation intervals in all WCSB cased-hole operations. Gamma ray log is combined with the CCL in the GR-CCL combination log for WCSB cased-hole depth correlation; the GR channel identifies formation lithology independently of depth measurement, allowing the collar count to be verified against known formation marker depths from the original open-hole log. Perforating gun position in WCSB Devonian and Cretaceous cased-hole completions is controlled by the CCL offset from the nearest verified collar; misperforating by one casing joint (40 m) typically means the gun fires opposite the wrong formation, creating water influx or communication with an adjacent zone. Coiled tubing conveys the CCL sub as part of the plug-and-perf BHA in WCSB Montney and Duvernay horizontal completions; CT encoder depth plus CCL collar count provide the dual-channel depth verification required by WCSB operators before authorizing each plug set and gun fire event. Collar lock anchors the wireline or CT tool string at a specific casing collar verified by the CCL; the two devices work together to position and mechanically secure perforating guns or bridge plugs within the tight depth tolerances required for WCSB multi-zone completions.